Light olefins from HDPE cracking in a two-step thermal and catalytic process

The continuous catalytic pyrolysis of high density polyethylene (HDPE) has been carried out in a two-step reaction system involving a pyrolysis conical spouted bed reactor followed by a catalytic fixed bed reactor. The good performance of the conical spouted bed reactor has allowed using a low temperature in thermal cracking (500 °C) without defluidization problems, obtaining a volatile stream with a 90 wt.% overall yield of C12–C20 and waxes in the first step. The effect of the second-step operating conditions on product yields and composition has been studied using a catalyst based on a HZSM-5 (SiO2/Al2O3 = 30) zeolite. The influence of temperature in the 350–550 °C range and space–time in the 0–8 gcat min gHDPE−1 range has been studied. An increase in temperature or space–time gives way to an increase in the yield of light olefins, reaching a value of 62.9 wt.% at 550 °C and 8 gcat min gHDPE−1, with the individual yields of ethylene, propylene and butenes being 10.6, 35.6 and 16.7 wt.%, respectively. Although the yield of single-ring aromatics increases when both variables studied are increased, the maximum yield obtained was lower than 13 wt.%. The yield of waxes (the main product in the first step) is negligible even at low temperatures or spaces-times, which evidences the efficiency of the catalytic step.

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• The two-step reaction system allows working with a continuous feed of HDPE.
• The product stream obtained in the pyrolysis step is mainly composed of waxes.
• The low residence time in the catalytic reactor avoids by-product formation.
• High yield of C2–C4 olefins (62.9 wt.%) at 550 °C and 8 gcat min gHDPE−1.